US Senator Scott Brown today criticized the state’s welfare department for sending voting registration forms to 478,000 people on public assistance, saying the mass mailing was a ploy to boost the ranks of Democratic voters and benefit rival Elizabeth Warren’s campaign.

The state’s Department of Transitional Assistance last month sent registration forms, along with prepaid return envelopes, as part of a settlement over a lawsuit accusing the Patrick administration of violating the federal “motor voter” law.

It requires states to provide voter registration at motor vehicle and public assistance offices.

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The suit was filed in May by a number of voting rights groups, including Demos, a left-leaning group based in New York whose board is chaired by Warren’s daughter, Amelia Warren Tyagi.

Voting rights groups have sued a number of states for failing to comply with the motor votor law, most recently Pennsylvania. Cases have been settled in New Mexico, Indiana, and Georgia.

The state denied that the mailings were politically motivated, but Brown’s campaign said they were “clearly designed” to benefit Warren’s campaign. The Harvard Law School professor is challenging Brown as he seeks reelection to the seat he won for the Republicans in a January 2010 special election.

“I want every legal vote to count, but it’s outrageous to use taxpayer dollars to register welfare recipients as part of a special effort to boost one political party over another,” the senator said in a statement. “This effort to sign up welfare recipients is being aided by Elizabeth Warren’s daughter and it’s clearly designed to benefit her mother’s political campaign.”

The state spent $276,000 on the mailings, which the Boston Herald reported in today’s edition.

Warren’s campaign dismissed Brown’s accusations.

“For Brown to claim this is some kind of plot against him is just bizarre,” said campaign manager Mindy Myers, adding that the attack on Warren’s daughter was “ridiculous.

“His entire attack is built on efforts in multiple states to enforce a law passed almost 20 years ago with bipartisan support. Even the Bush Justice Department filed suit to enforce this provision of that law,” said Myers.

Daniel Curley, commissioner of the state’s transitional assistance department, said the mailings had nothing to do with partisan politics.

“We’re responding to a court decision,” he said.

Peter Schworm can be reached at schworm@globe.com. Follow him on Twitter @globepete.